To reproduce this with new empty scene, if anyone is interested to test :
1. Import character controller package from standard assets.
2. Add First Person Controller to scene.
3. Delete old Main Camera, because it's not needed anymore.
4. Add 3D Object - Plane to scene under First Person controller, so you can move around without falling.
5. Scale that plane big enough so you have more room to move.
6. Add UI - Canvas
7. Change Canvas Render Mode to World Space
8. Move, resize and scale canvas, so it nicely fit to camera view
9. Add UI - Button under canvas
10. Make button Highlighted color something easy to notice (red etc.)
11. Hit play and move around.

You will notice that button get Highlighted color randomly even when mouse cursor is not over the button.